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Monday, 17 July 2017

In flower: Solanum pyracanthos

S.pyracanthos

A couple of years ago I obtained at Malvern Flower Show a
plant of Solanum pyracanthos, the
Porcupine Tomato. The reason for the name is pretty obvious – it is heavily
armed on the stem and even the leaves with serious spikes.

Solanum species include some of the world’s major food crops,
including potato, tomato and eggplant, as well as crops of more local
significance such as naranjilla. Not in the same genus, but still part of the
Solanaceae, are the peppers (Capsicum) and various species of Physalis such as
the tomatillo.

Note spines on leaves

The Solanaceae as a group often produce potent alkaloid
toxins in their foliage to protect against herbivores, and to humans at least
many wild species have toxic fruit. As a result Europeans were slow to take to
using plants like potatoes or tomatoes after they were first introduced,
although they came round eventually.

The Solanaceae have around 3,000- 4,000 species in around 90
genera, and a worldwide distribution. Perhaps 1,400 species are currently in
Solanum, many in South America, but there are other centres of endemism in
North America, Africa, Australia, and Madagascar, the home of S.pyracanthos.

With so many species, Solanum is divided into numerous
subgenera and sections. Solanum pyracanthos belongs to section Melongena, the
same group as the eggplant, a group mainly confined to Africa. The numerous
Madagascan species of Solanum seem to be part of a mainly endemic radiation of
this section. Although Madagascars native animals, especially the lemurs, are
the most famous targets of conservation concern, but the plants are just as
significant and indeed endangered – the massive deforestation of the island in
the last 2,000 years or so since human beings colonised the island must have
had a major impact. Loss of native plants would have had an equally massive
knock on effect on the native invertebrate fauna, and very little published
work is available on these.

The range and habitat of S.pyracanthos
in its native range is unclear, but its impressive appearance means that it has
been cultivated in many places. Although not frost resistant, in warmer
climates it grows easily and can become an invasive weed. It appears to be a
fairly typical “weedy” successional species spread by birds or mammals eating
the fruit – which is allegedly technically edible although extremely
distasteful to humans (check carefully first before attempting one).

Archaeolemur edwardsii

The amount of armament covering the stems and even leaves is
plainly a deterrent against plant eating animals. What they might be is not
certain. Those living lemurs which subsist mainly on foliage spend most of
their time in trees and seldom come down to ground level, and S.pyracanthos seldom reaches more than
1m tall. Some extinct lemurs such as Archaeolemur
were more terrestrial however, and several living species of lemur are
surprisingly resistant to toxins. It seems more likely that the main threats
were animals like the now extinct giant tortoises such as Aldabrachelys abrupta, a close relative of the living Aldabra Giant
Tortoise.